The long-term goal of this proposal is to develop an understanding of intralimb and interlimb coordination useful for diagnosing and rehabilitating functional deficits in neurologically impaired individuals. Past work has focused on developing computer simulation models of lower-limb motor tasks to understand how biomechanical constraints affect muscle coordination. Current work focuses on pedaling, a paradigm that, like walking, allows conceptually rich split treadmill type experiments, but, unlike walking, permits full control of task mechanics and interlimb relationships. The investigators have found that normal two-legged pedaling is achieved by coordinating muscles to cyclically execute three antagonistically paired biomechanical functions (flexor/extensor, anterior/posterior, plantarflexor/dorsiflexor). In one-legged pedaling tasks, the coordination of these functions is significantly different, even when the loading is identical to the loading experienced during two-legged pedaling. The two-legged coordination pattern is restored when the contralateral leg generates rhythmic force and is moved antiphase to the ipsilateral leg. Results suggest that excitatory and/or inhibitory interlimb influences operate between a contralateral biomechanical function and the homologous antagonistic function in the ipsilateral leg. Thus the generation of motor output in one limb is inherently bilateral: it depends on the sensorimotor state of the contralateral limb. The focus of this proposal is to understand how the inherent bilateral control mechanisms change the motor output of a pedaling leg when, unlike in normal pedaling, the contralateral leg executes a biomechanical function that is not the homologous antagonist to the ipsilateral function. Experiments will be performed on a split-axis pedaling ergometer. The subject will pedal with the ipsilateral leg against the same mechanical load in all experiments, namely, the load it encounters during normal two-legged pedaling, emulated by a servomotor. The subject is asked to produce propulsive crank forces with the contralateral leg as it is moved by a second servomotor to maintain one of the following relationships to the ipsilateral leg: constant phase offset, opposite direction of rotation, different cyclic frequency, or different path length. Contributions of individual muscles, particularly the biarticular thigh muscles, to the six biomechanical functions will be analyzed in each leg. Computer simulations of each leg will be performed to emulate the kinematic, kinetic, and EMG data. The results will be synthesized in the form of a bilateral motor control model of rhythmic locomotor behavior.